S4 collection of fine-mapping fits for one or more QTL
studies on a single LD block. Keyed by the identity tuple
(study, context, trait, method); each entry is a
FineMappingRow.
Required columns: study, context, trait,
method, entry. The 4-tuple is unique. The caller is
expected to construct one QtlFineMappingResult per LD block (no
in-class block indexing).
Optional columns jointStudies, jointContexts,
jointTraits appear when the collection contains rows produced by a
jointSpecification-driven joint fit, carrying the semicolon-joined
member identities of the joined axis.